Capacitance transducers of varying types have been used extensively to sense motion resulting from rotation, translation, deformation, or a combination thereof. Most of the capacitance transducers which appear in the prior art, however, do not possess sufficient range, linearity, or sensitivity to be useful in a variety of engineering applications. An exception, in the case of rotation, is the linear rotary differential capacitance (LRDC) transducer described in U.S. Pat. No. 5,028,875. The present invention describes a symmetric differential capacitance (SDC) transducer for accomplishing similar objectives as the LRDC transducer, but in the realm of translation and deformation. With regard to electrical operation, the novel SDC units of this invention are similar to those of the LRDC design; with regard to mechanical configuration and application, however, the SDC units are unique and different.
The trend in capacitive transducer manufacture is toward even smaller units. When used in micro-electro-mechanical devices, transducers of the prior art (two active components) are expected to experience significant performance degradation because of extraneous stray capacitance and scaling difficulties. One means of compensating for this degradation is to use computational electronics that is external to the sensor, such as described in U.S. Pat. No. 5,028,876. A better way is to employ a transducer which automatically compensates as a natural consequence of its intrinsic properties. When operated with synchronous detection, the SDC transducer of this invention provides this automatic compensation.
As with the LRDC design, the most fundamental SDC design is one in which wires are attached to the electrodes. Because motion transducers necessarily have moving parts, and because the attachment of wire leads to the moving part of the transducer can result in undesirable damping; alternative configurations are known in which there are no wires or brushes which contact the moving member.
Differential capacitance transducers of the prior art have almost universally utilized only two active components. In response to the motion being sensed, one capacitance increases while the other capacitance decreases. Examples are found in U.S. Pat. Nos. 3,278,919; 3,729,991; 4,227,182; 4,310,806; 4,386,312 and 4,389,646 to which reference is made for further background of this invention. The pair of capacitances may be part of a bridge circuit, if desired. If this prior art transducer is part of a bridge, only two of the four components are variable, so that the circuit is referred to as a half-bridge.
With the trend toward smaller devices, and the ultimate goal of micro-sized capacitive transducers on silicon; it has been recognized that stray capacitances can be very detrimental to performance. One way to provide better immunity to these stray capacitances was noted above. Another has been indicated in U.S. Pat. No. 4,403,219, where a current coupled input is used to reduce the indicator error due to stray capacitance.
A number of previous inventions have utilized electrode geometries that are similar to the present invention. An example is U.S. Pat. No. 3,151,239. Its concern is with different objectives than the present invention, and the electronics is significantly different Another example is U.S. Pat. No. 4,303,919 (FIGS. 1-6, where geometry selection was also motivated by different objectives than the present invention. That invention concerned with monitoring multi-component motion, not detection advantages born of symmetry.